The big green 'unread' button in the top-right displays all topics that received updates since your last visit, clicking on the green 'new' button right of the topic title brings you to the first new contribution in the topic.

Yes, that's actually what it does, shows unread since last visit, if you log out. If you don't log out it can be used as a refersh unread topics.

However after hitting the green unread button look at the lower right of the results page. You will find another button that says "All Unread Topics". Which will bring up every topic that has a new post since you last read it or marked everything as read.